As known, metal sheet and panel cutting centres are systems intended for cutting planar surfaces, having sometimes a very complex perimeter, starting from a typically rectangular sheet (for example measuring 1500×3000 mm with a thickness ranging from 0.5 to 20 mm). The cut is performed employing various techniques, also based on the composition of the sheet material: for example, there are machines employing laser cutting, oxygen lance cutting, water-jet cutting, plasma cutting and so forth.
A particular sector is that of metal sheet cutting, which is typically performed employing laser devices.
In this sector, in the loading and unloading phase to and from the cutting centre, there is the need to handle sheets of a non-negligible weight. Typically, sheets are stored in a warehouse, where they are stacked, for example according to even thicknesses, wherefrom they must be taken and laid into a collection point in the order in which they must be used (hence also with different thicknesses). From the collecting point, the loading/unloading system must individually take the virgin sheets and lay them onto the movable tray of the cutting centre. Once the cutting is complete, the loading/unloading system must again take the cut pieces and forward them to unloading and palletising.
The peculiarity of this process lies in the fact that the virgin sheet and the cut pieces are very different products and hence require a lifting and handling system which is capable of handling at the same time two rather different work conditions.
Moreover, the cut pieces are rarely repetitive in size and arrangement between one sheet and the next.
In order to meet these requirements, it is possible to use two different techniques. The most commonly used technique provides to arrange a bridge-like castle framework, opposite the cutting centre, whereon both a raiser with suction-cup gripper—which is intended to handle the virgin sheet—and a forklift—which is intended to take the cut-in sheet and to transfer it into the unloading station—are translatable. An example of such system is the ACS model available from Antil SpA.
This solution is very popular, also due to its simplicity and ease of control, but it implies some drawbacks. The main problem is due to the fact that the cut-in sheet cannot be fully disassembled into individual components, otherwise the forklift might not operate correctly. This implies that the cutting programme does not complete any of the cuts of the individual pieces, but always leaves one or more small connecting bridges having a common frame (which then makes up work off-cuts). Therefore, once the cut-in sheet has been unloaded, it is necessary to break these small connecting bridges to free the individual pieces from the off-cuts: this task is traditionally performed by hand, with the imaginable consequences in terms of cost and performance time.
Moreover, it is necessary that the suction loading system consists of a large number of small vacuum cups, otherwise it would be unable to impart an adequate force for lifting the weight of thick metal sheets.
A second technology which may be employed in a loading/unloading system is that of Cartesian mechanical hands, which is traditionally used also in a number of other similar sectors.
A Cartesian robotized hand typically consists of a retaining member, for example a vacuum cup, mounted at the end of a support mounted movable on a pair of bridge cranes: by suitably controlling driving motors, it is possible to move and position the retaining member into a desired point X, Y of the plane.
In the cases in which the retaining member is also vertically translatable on the movable support, it is possible to obtain the desired positioning along the Z axis, thereby achieving an overall positioning in the Cartesian space X, Y, Z.
A suitably-sized and suitably-configured Cartesian mechanical hand is hence suited to the handling of sheets also. The advantage of a suitably controlled mechanical hand is that of being able to perform a simple, repetitive translating movement, useful for the loading of a virgin sheet, but also of performing more complex movements, such as the identification of a single workpiece cut out from the sheet, the lifting thereof and the transfer thereof into a desired unloading position.
For transferring a large and heavy object, such as a metal sheet, it is also widely known to resort to the coordinated (i.e. synchronous) movement of two or more Cartesian mechanical hands, so that it is not necessary to use a single, very sturdy mechanical hand which must necessarily act in the centre of gravity of the object.
As can be guessed, resorting to Cartesian mechanical hands makes the system more flexible, even though it causes more problems in terms of tuning.
In particular, the workpieces may be completely cut and separated from the cut-off structure, because they are then taken individually by the mechanical hands and orderly stacked in the unloading station, which makes useless the subsequent manual separation action. Again, as known, such a system is by its nature suited to operate unmanned continuously for many hours.
The present invention fits in this sector.
In particular, the object of the invention is that of providing a loading/unloading system with Cartesian mechanical hands which is particularly efficient and suited to meet the peculiar requirements in the handling of metal sheets serving cutting centres.
As a matter of fact, in this field, as well as in similar fields, the handling system normally provides a single bridge crane whereon a single Cartesian mechanical hand is supported translatable, or a pair of mechanical hands, equipped with suction-cup ends: this configuration is not fully satisfactory for transferring large and heavy workpieces, such as full virgin metal sheets or cut-off structures, or to have a good unloading productivity of the cut workpieces. Furthermore, when it is necessary to exploit simultaneously both mechanical hands for lifting a large piece, it is no longer possible to change the orientation thereof in the plane, because the pair of mechanical hands remains necessarily aligned on the same bridge crane.
Moreover, the suction-cup gripping elements are unable, individually, to lift heavy loads: it is hence necessary to make a compromise choice between a large number of suction cups—which, however, increases the bulk of the gripping head of the hand—and a small number of suction cups, which, however are unable to lift heavy pieces. It must furthermore be noted that the unloading of the cut-off structure is critical, because it does not offer a surface wide enough to allow the suction cups to operate correctly: there is hence a significant limit to said structure weight.
Finally, there is a problem connected with the feeding tray of the handling centre: as a matter of fact, it should be made of sturdy metal materials, to meet the requirements related to the handling apparatus, but at the same time it is intended to interact with the laser cutting machine within which it might undergo accidental welding to the sheets to be cut.
Therefore, the Applicant set himself the object of solving these drawbacks of the known art.